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The Saglek-Hebron Complex located in Northern Labrador, Canada, is one of the oldest granite-greenstone terrains on Earth. It is dominated by granitoids as old as 3.9 Ga and several enclaves of supracrustal rocks. We present here the largest whole-rock major and trace element geochemical dataset (over 100 samples) on the mantle-derived rocks from the Saglek-Hebron Complex to constrain their petrogenesis. The mafic amphibolites are metavolcanic rocks basaltic in composition, have tholeiitic affinities, and exhibit relatively homogeneous compositions with flat rare-earth element profiles. They include incompatible element enriched and depleted compositions, both which appear to be related by fractional crystallization of a gabbroic assemblage within volcanic flows. The enriched mafic rocks exhibit higher Ti contents compared to the more depleted mafic rocks, representing, respectively, evolved liquids and pyroxene-rich cumulate-liquid mixtures. Previous work has suggested the occurrence of two distinct supracrustal units in the Saglek-Hebron Complex, the Eoarchean Nulliak assemblage and the Mesoarchean Upernavik assemblage. Here we report that the petrological and geochemical composition of both assemblages is indistinguishable. This suggests either that the Nulliak and Upernavik metavolcanic rocks represent a single unit, or that they were formed by identical processes in a comparable context, despite an age difference of ∼400 Ma. Contrary to some other early metavolcanic belts, such as the Isua supracrustal belt and Nuvvuagittuq greenstone belt, none of the metavolcanic rocks from the Saglek-Hebron Complex studied here exhibit geochemical compositions that could be reminiscent of suprasubduction environments. The Saglek-Hebron Complex also includes two compositionally distinct groups of ultramafic rocks characterized by different Al/Ti ratios, Fe contents and controlled by olivine fractionation with different Fo contents. Ultramafic rocks from both groups appear to be cumulates derived from distinct parental magmas with a komatiitic basalt composition. One of these parental magmas may be genetically linked through fractional crystallization with the mafic metavolcanic rocks, whereas the other group is more difficult, at least geochemically, to relate to the Saglek-Hebron basaltic rocks.

International audience

Southern West Greenland contains some of the best-studied and best-preserved magmatic Eoarchean rocks on Earth, and these provide an excellent vantage point from which to view long-standing questions regarding the growth of the earliest continental crust. In order to address the questions surrounding early crustal growth and complementary mantle depletion, we present Laser Ablation Split Stream (LASS) analyses of the U–Pb and Hf isotope compositions of zircon from eleven samples of the least-altered meta-igneous rocks from the Itsaq (Amitsoq) Gneisses of the Isukasia and Nuuk regions of southern West Greenland. This analytical technique allows a less ambiguous approach to determining the age and Hf isotope composition of complicated zircon. Results corroborate previous findings that Eoarchean zircon from the Itsaq Gneiss (∼3.85 Ga to ∼3.63 Ga) were derived from a broadly chondritic source. In contrast to the Sm–Nd whole rock isotope record for southern West Greenland, the zircon Lu–Hf isotope record provides no evidence for early mantle depletion, nor does it suggest the presence of crust older than ∼3.85 Ga in Greenland. Utilizing LASS U–Pb and Hf data from the Greenland zircons studied here, we demonstrate the importance of focusing on the magmatic (rather than detrital) zircon record to more confidently understand early crustal growth and mantle depletion. We compare the Greenland Hf isotope data with other Eoarchean magmatic complexes such as the Acasta Gneiss Complex, Nuvvuagittuq greenstone belt, and the gneissic complexes of southern Africa, and all lack zircons with suprachondritic Hf isotope compositions. In total, these data suggest only a very modest volume of crust was produced during (or survived from) the Hadean and earliest Eoarchean. There remains no record of planet-scale early Earth mantle depletion in the Hf isotope record prior to 3.8 Ga.

International audience; Recycling of crust into the mantle has left only small remnants at Earth’s surface of crust produced within a billion years of Earth formation. Few, if any, of these ancient crustal rocks represent the first crust that existed on Earth. Understanding the nature of the source materials of these ancient rocks and the mechanism of their formation has been the target of decades of geological and geochemical study. This traditional approach has been expanded recently through the ability to simultaneously obtain U-Pb age and initial Hf isotope data for zircons from many of these ancient, generally polymetamorphic, rocks. The addition of information from the short-lived radiometric systems 146Sm-142Nd and 182Hf-182W allows resolution of some of the ambiguities that have clouded the conclusions derived from the long-lived systems. The most apparent of these is clear documentation that Earth experienced major chemical differentiation events within the first tens to hundreds of millions of years of its formation, and that Earth’s most ancient crustal rocks were derived from these differentiated sources, not from primitive undifferentiated mantle. Eoarchean rocks from the North Atlantic Craton and the Anshan Complex of the North China Craton have sources in an incompatible-element-depleted mantle that dates to 4.4-4.5 Ga. Hadean/Eoarchean rocks from two localities in Canada show the importance of remelting of Hadean mafic crust to produce Eoarchean felsic crust. The mafic supracrustal rocks of the Nuvvuagittuq Greenstone Belt are a possible example of the Hadean mafic basement that is often called upon to serve as the source for the high-silica rocks that define continental crust. Many, but not all, ancient terranes show a shift in the nature of the sources for crustal rocks, and possibly the physical mechanism of crust production, between 3.0-3.6 Ga. This transition may reflect the initiation of modern plate tectonics. Eoarchean/Hadean rocks from some terranes, however, also display compositional characteristics expected for convergent margin volcanism suggesting that at least some convergent margin related magmatism began in the Hadean. The persistence of isotopic variability in 142Nd/144Nd into the mid-Archean, and the eventual reduction in that variability by the end of the Archean, provides new information on the efficiency by which mantle convection recombined the products of Hadean silicate-Earth differentiation. The rate of crust production and recycling in the Hadean/Archean, however, is not resolved by these data beyond the observation that extreme isotopic compositions, such as expected for Hadean evolved, continent-like, crust are not observed in the preserved Eoarchean rock record. The lack of correlation between 142Nd/144Nd and 182W/184W variation in Archean rocks suggests that these two systems track different processes; the Sm-Nd system mantle-crust differentiation while Hf-W is dominated by core formation. The major silicate differentiation controlling Sm/Nd fractionation occurred at ∼4.4 Ga, possibly as a result of the Moon-forming impact, after the extinction of 182Hf.

The Isua supracrustal belt (ISB) and the Nuvvuagittuq greenstone belt (NGB) are among the oldest suites of mafic volcanic rocks preserved on Earth and are the best candidates for representing its early crust. Despite the possible 500 Ma age difference between the belts, these mantle-derived rocks show compositional similarities, with features resembling rocks formed in subduction initiation environments. With the addition of new 142Nd data for the Garbenschiefer unit of the ISB reported here, high precision 142Nd data are now available for all the mafic lithologies from both belts. Mantle-derived rocks from both the ISB and NGB belts exhibit a range of 142Nd/144Nd ratios. The datasets for the two belts, however, are significantly different, suggesting a different origin for their 142Nd anomalies. Nearly all ISB samples have excesses in 142Nd, including the newly analyzed Garbenschiefer boninitic amphibolites (mean of + 12 ppm ). Excesses in 142Nd/144Nd compared to the Nd standard for all the ISB rocks range between +8 and + 20 ppm , with a near Gaussian distribution around + 12 ppm . This distribution could simply reflect the analytical error (±5 ppm) around a single 142Nd/144Nd ratio indicating that the samples formed after the extinction of 146Sm from a source with a nearly uniform 142Nd/144Nd ratio. In contrast, the NGB shows a range of 142Nd/144Nd ratios from +8 to − 18 ppm relative to the modern Nd standard and displays a flat distribution of 142Nd/144Nd ratios. The ISB samples show no significant correlation between their 142Nd/144Nd and Sm/Nd ratios, consistent with their formation in the Eoarchean via melting of a Hadean depleted mantle. In contrast, all NGB samples display a 142Nd/144Nd vs. Sm/Nd correlation, consistent with their crystallization in the Hadean. The mantle sources for both the ISB and NGB mantle-derived rocks have a similar 142Nd/144Nd ratio at the possible age of formation of the NGB (∼4.3 Ga) suggesting the derivation of ISB and NGB rocks from a common early-formed depleted mantle source formed between 4.47 and 4.42 Ga with a 147Sm/144Nd ratio ∼0.218. This mantle appears to have been an important source component involved in the formation of the primitive crust during most of the Hadean and Eoarchean eons.

Tungsten concentration and isotopic data, coupled with highly siderophile element (HSE) concentration and Os isotopic data for = 3.66 billion year-old ultramafic, mafic, and felsic supracrustal rocks from the Nuvvuagittuq Greenstone Belt, were investigated to place additional constraints on the nature and origin of W-182 heterogeneities in the early Earth. The absolute and relative abundances of HSE in the mafic and ultramafic rocks are generally similar to those in modern rocks with comparable MgO contents. Further, most samples plot close to 3.8 to 4.4 Ga reference lines on a Re-187-Os-187 isochron diagram, indicating that HSE abundances in most Nuvvuagittuq samples remained undisturbed by post-Eoarchean metamorphic events. All Nuvvuagittuq samples analyzed show well-resolved W-182 excesses, ranging from + 6 to + 17 ppm, compared with the modern isotopic composition of W. The observed level of HSE abundances, coupled with the W-182 enrichments of these rocks is seemingly inconsistent with their derivation from mantle that was isolated from a HSE-rich and W-182-depleted late accretionary component. However, the absence of correlation between W and MgO contents, as well as variable W enrichment relative to elements with similar incompatibilities suggest that the W in the Nuvvuagittuq samples involved fluid transport of the W in either the crust or the mantle, and that it has little genetic relationship with the HSE. Given the lack of evidence for extensive redistribution of W in the crust, the HSE and W elemental and isotopic systematics of the Nuvvuagittuq rocks may be explained by a model whereby peridotitic mantle, with modern-like HSE abundances, was metasomatized by fluids derived from a W-182-rich crustal component that had been recycled into the mantle via subduction or delamination. The source of the W-182 excess carried by this crustal component remains enigmatic. It was most likely inherited from either pre-late accretionary, or early-depleted parental mantle reservoirs. (C) 2014 Elsevier B.V. All rights reserved.

of the belt formed at � 4·28 Ga, which would make it the only known remnant of Hadean crust preserved on Earth. The dominant lithology of the belt has a mafic composition that consists of gneisses ranging from cummingtonite amphibolite to garnet^biotite schist composed of variable proportions of cummingtonite þ biotite þ quartz, � plagioclasegarnetanthophyllitecordierite. The composition of this unit ranges from basalt to andesite and it is divided into two distinct geochemical groups that are stratigraphical- ly separated by a banded iron formation (BIF). At the base of the se- quence, the mafic unit is mainly basaltic in composition and generally has relatively low Al2O3 and high TiO2 contents, whereas above the BIF, the unit is characterized by high Al2O3 and low TiO2 contents and exhibits a wider range of compositions from bas- altic to andesitic. The low-Ti unit can be further subdivided into a trace element depleted and a trace element enriched subgroup. The high-Ti unit is characterized by relatively flat REE patterns as opposed to the low-Ti gneisses, which display light REE-enriched profiles with flat heavy REE slopes. The incompatible element depleted low-Ti rocks have U-shaped REE profiles.The geochemical groups have compositional analogues in three types of ultramafic sills that exhibit the same stratigraphic succession. Generally, the mafic gneisses have low Ca, Na and Sr contents, with many samples having CaO contents51wt %. Such low Ca contents are unlikely to represent the original composition of their igneous precursors and are interpreted to reflect intensive alteration of plagioclase. These compositional characteristics along with the presence of cordier- ite þ anthophyllite suggest that the protoliths of the mafic gneisses were mafic volcanic rocks exhibiting variable degrees of hydrothermal alteration. The high-Ti compositional type shares geochemical char- acteristics with tholeiitic volcanic suites with low Al2O3 and high TiO2 contents and is consistent with crystal fractionation at low pressures under dry conditions. In contrast, the low-Ti compositional group is geochemically similar to boninitic and calc-alkaline volcanic suites.The high Al2O3 and lowTiO2 contents in the andesitic com- positions suggest the early crystallization of Fe^Ti oxides and late appearance of plagioclase, and are more consistent with fractionation at elevated water pressures. The succession from 'tholeitic' to 'calc-alkaline' magmatism seen in the Nuvvuagittuq greenstone belt is typical of the volcanic successions of many younger Archean green- stone belts. Regardless of the exact tectonic setting, this volcanic suc- cession suggests that the geological processes responsible for the formation and evolution of Archean greenstone belts were active at 3·8 Ga and perhaps as early as 4· 3G a.

Analysis of isotope-geochemical data obtained for the early crustal complexes of the Earth provided constraints on the formation time, scales of development, and geochemical features of protocrust. Most informative were isotope-geochemical and geochemical data on the oldest zircons with ages up to 4.4 Ga, short-lived 146Sm/142Nd isotope system, and lead isotope composition of the oldest rocks of Greenland. The presence of positive 142Nd anomaly in the rocks of West Greenland and negative anomaly in the amphibolites of the oldest Nuvvuagittuq greenstone belt of the Superior province (O’Neil et al., 2008) indicates the early differentiation of the Earth material into depleted mantle and enriched (basaltic) crust (Caro et al., 2006; Benett et al., 2007a, b; O’Neil et al., 2008). Pb-Pb isotopic systematics of the oldest crustal rocks from West Greenland and Labrador testifies that high μ enriched crust (238U/204Pb = 10.9) of basaltic composition already existed 3.9 Ga ago (Kamber et al., 2003). Based on isotope-geochemical and geochemical features of the oldest zircons in the Late Archean greenstone belts of the Yilgarn block (Western Australia), the crust of intermediate-felsic composition and water on the Earth’s surface already existed 4.4 Ga ago (Wilde et al., 2001).

Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4-2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered some of the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions (δ(33)S, δ(34)S, and δ(36)S) of the3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions (Δ(33)S up to +2.2‰) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in (33)S/(32)S, (34)S/(32)S, and (36)S/(32)S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earth's formation, a common mechanism for S-MIF production was established in the atmosphere.

International audience; The Nuvvuagittuq greenstone belt is dominated by mafic rocks, called the Ujaraaluk unit, that are mostly composed of cummingtonite-plagioclase-biotite with variable amounts of garnet. While the oldest zircons contained in thin intrusive trondhjemitic bands are ∼3.8 Ga, 146Sm-142Nd systematics suggest that the Ujaraaluk unit is as old as 4.4 Ga. The Nuvvuagittuq greenstone belt is surrounded by Eoarchean TTGs that have geochemical and isotopic compositions consistent with their derivation by partial melting of a source similar in composition and age to the Ujaraaluk unit. New zircon dates reported here show the Nuvvuagittuq TTGs to consist at least of four distinct age units of 3.76 Ga, 3.66 Ga, 3.5-3.4 Ga and 3.35 Ga. The Hf isotopic compositions of zircons from the TTG are consistent with derivation from Hadean mafic crust. The 3.66 Ga to 3.35 Ga TTGs appear to have been formed primarily from melting of a source compositionally similar to the 4.4 Ga Ujaraaluk unit, whereas the more radiogenic Hf of the zircons from the 3.76 Ga TTGs may suggest derivation from melting of a source compositionally similar to 4.1 Ga intrusive gabbros. Alternatively, the distinct rare earth element patterns of the 3.76 Ga and 3.66 Ga TTGs suggest their derivation from sources with variable amounts of residual garnet and hence formation at different depths. The composition of the older TTGs is indicative of a deeper source that may have involved a greater interaction between the melt and the mantle to explain the more radiogenic Hf isotopic compositions of their zircons. Sources compositionally similar to the Ujaraaluk unit and intrusive gabbros appear to be the most likely candidates for the Hadean precursor of the Nuvvuagittuq TTGs.

International audience; Mafic igneous rocks of the Nuvvuagittuq Greenstone Belt (NGB) crystallized before 3.8 Ga and possibly as early as 4.3 Ga, potentially making the belt the oldest known supracrustal sequence on Earth. However, detrital zircons from a rare quartz-biotite schist in the NGB yield significantly younger ages of ≈3.77 Ga or less. These appear to be inconsistent with the ages of the mafic igneous rocks, as the quartz-biotite schist has been interpreted as a metaconglomerate, formed by the dismantling of preexisting lithologies. In order to assess this genetic interpretation, we performed a sulfur and oxygen isotope study of the quartz-biotite schist. Sulfide grains found in quartz clasts and the matrix show significant mass-independent fractionation of sulfur isotopes (+0.2‰≤Δ33S‰≤+1.0‰; mean Δ33S=+0.5±0.1‰). Secondary sulfides from crosscutting veins do not show mass-independent fractionation of sulfur isotopes (-0.1‰≤Δ33S≤+0.3‰; mean Δ33S=+0.1±0.1‰). Oxygen isotope compositions of quartz from clasts, matrix and a fine-grained lens are highly enriched in 18O (16.9‰≤δ18O≤26.7‰). Non-zero Δ33S values indicate a surficial origin for sulfur, probably the Eoarchean atmosphere, while high δ18O values suggest a low-temperature (65±18 °C) origin for the quartz, likely as chemical precipitation of a chert precursor from Si-saturated seawater. Therefore, the coupled S- and O-isotope measurements show that primary isotopic signatures characteristic of surficial environments survived the protracted metamorphic history of the NGB, and suggest that the quartz-biotite schist contains material that originated as chemical metasediments. The near mono-mineralic compositions of the clasts (quartz) and their shared 18O-enrichment suggest that they had a common protolith, which was deposited prior to the formation of the schist, and subsequently reworked. Whether the quartz-biotite schist represents a metaconglomerate or a structural melange, it preserves remnants of some of the oldest chemical sediments on Earth.

Neodymium-142 data for rocks from the Nuvvuagittuq greenstone belt in northern Quebec, Canada, show that some rock types have lower 142 Nd/ 144 Nd ratios than the terrestrial standard (ϵ 142 Nd = –0.07 to –0.15). Within a mafic amphibolite unit, 142 Nd/ 144 Nd ratios correlate positively with Sm/Nd ratios and produce a 146 Sm- 142 Nd isochron with an age of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(4280_{-81}^{+53}\) \end{document} million years. These rocks thus sample incompatible-element-enriched material formed shortly after Earth formation and may represent the oldest preserved crustal section on Earth.

Publisher Summary This chapter describes the geology of the 3.8 Ga Nuvvuagittuq Greenstone belt, Northeastern Superior province, Canada. The Nuvvuagittuq belt is a volcano-sedimentary succession that occurs as a tight to isoclinal synform refolded into a more open south-plunging synform, with bedding largely parallel to the main, steeply dipping schistosity. The supracrustal assemblage of the belt is essentially composed of three major lithological units. The Nuvvuagittuq belt is surrounded by a 3.6 Ga tonalite, itself surrounded by a younger 2.75 Ga tonalite. Cummingtonite-amphibolites are the predominant lithologies of the Nuvvuagittuq greenstone belt. These peculiar amphibolites are dominated by cummingtonite, which gives this lithology a light gray to beige color, rather than the dark green to black color characteristic of hornblende-dominated amphibolites typical of the Superior Province. The striking feature of the western limb of the Nuvvuagittuq belt is the presence of numerous ultramafic and gabbroic conformable bodies within the faux-amphibolite.

Andreasen and Sharma raise concerns about the neodymium-142 data and age that we reported for rocks from the Nuvvuagittuq greenstone belt in Quebec, Canada. We agree that the issue of accurate mass fractionation correction is important, but stand by our discussion of this issue in our original report and our conclusion that the variation in 142 Nd/ 144 Nd ratios reflects the decay of 146 Sm caused by Sm-Nd fractionation within 300 million years of Earth’s formation.